1. Field of the Invention:
This invention relates to an improved method for the manufacture of cast aluminum articles from Al-Si-Mg type casting alloys.
The term "Al-Si-Mg type casting alloys" as used herein means the Al-Si-Mg alloys of the type corresponding to those designated as AC4A and AC4C by Japanese Industrial Standard for Aluminium Alloy Castings (JIS H5202). Since these Al-Si-Mg type casting alloys have good casting properties, possess appropriate strength and toughness, and excel in weldability, they are widely used for parts for engines in automobiles and ships and other machine parts.
2. Description of the Prior Art:
Heretofore in the manufacture of cast articles of these alloys, the alloys after melting and pouring into molds have been subjected to a solid solution heat treatment and then to quenching and an artificial aging heat treatment in order to acquire the necessary strnegth and toughness. The solid solution heat treatment has to be carried out at elevated temperatures within the range of from 500.degree. to 550.degree. C. for long periods ranging from several hours to ten or more hours. Further, the aging step needs to continue at temperatures within the range of from 140.degree. to 200.degree. C. for similarly long periods ranging from several hours to ten or more hours. These treatments have necessarily affected the casting operation adversely notably from the standpoints of production efficiency and heat economy.
With a view to avoiding these disadvantages involved in castings of the type requiring thermal treatment, there has been proposed a so-called "solutioning casting method" which, as disclosed in Japanese Patent Publication No. 10901/1962, for example, comprises quenching the casting body to room temperature immediately after casting, thereby simultaneously effecting quenching and formation of supersaturated solid solution, and subsequently subjecting the shaped alloy body to the artificial aging treatment.
However, when this "solutioning casting method" is applied to alloys having the composition contemplated by the present invention, the ultimate cast articles obtained after the aging treatment lack sufficient strength and toughness as compared with the products of the so-called T6 material which have undergone conventional aging subsequently to conventional solution heat treatment.